Surface shape measuring system

ABSTRACT

A grating ( 3 ) is disposed to face the measurement target surface of a measurement target object ( 11 ). A light source ( 1 ) irradiates the grating ( 3 ) with illumination light. A camera ( 6 ) captures a moire fringe image formed on the grating ( 3 ) by light passing through the grating  3  and reflected by the measurement target surface. A moving means ( 9 ) changes the distance (H) between the grating  3  and the measurement target surface. An analyzing means ( 8 ) performs an analysis process of obtaining 3-D shape information of the measurement target surface from the image picked up by the camera ( 6 ) in at least two cases in which the distance (H) is set to different values, and obtains, on the basis of the 3-D shape information in each case and the distance H, true 3-D shape information from which the measurement error caused by the inclination of the measurement target surface is eliminated.

1. TECHNICAL FIELD

The present invention relates to a surface shape measuring system whichmeasures the surface shape of a relatively flat object such as a compactdisk, magnetooptical disk, or hard disk.

2. BACKGROUND ART

Storage media capable of high-density storage such as optical disks arerecently in great use. They are required to have high flatness in orderto achieve a higher storage density. For this purpose, the surface shapeof a storage medium must be checked in a manufacturing process. As amethod of performing such surface shape measurement, a moire method hasbeen known. The moire method is a method of measuring the surface shapeof an object from the moire fringes (contour lines of a surface shape)produced by superimposing a grating and a grating image deforming inaccordance with the shape of the object when light from a point lightsource passes through the grating and strikes the object.

In the moire method using divergent light, however, the contour lineinterval (a level difference per moire fringe) increases with anincrease in the distance between a grating and an object. This causes anerror unless the ordinal number (degree n) of a given moire fringe fromthe grating surface can be specified. In addition, in the moire method,since only contour lines are displayed, recesses and projections cannotbe discriminated. The measurement precision can be improved by reducingthe pitch of the grating. If, however, the pitch is reduced, thecontrast of the moire fringes decreases. This limits the contour lineinterval to about 10 μm at most.

In order to solve such a problem, therefore, a scheme based on acombination of a parallel light moire method and a phase shift methodhas been proposed (e.g., Japanese Patent Laid-Open No. 7-332956). Acharacteristic feature of the parallel light moire method is that lightfrom a point light source 21 is converted into parallel light by using alens 22 to always make the contour line interval constant regardless ofthe distance from a grating 23, as shown in FIG. 8. For this reason,there is no need to determine a degree n of a moire fringe, and no errorbased on the contour line interval is caused. When reflected light isused, a contour line interval Δh is obtained by only an incident (exit)angle θ of light and a pitch p of the grating 23 according to thefollowing equation:Δh=p/(2 tan θ)  (1)Referring to FIG. 8, reference numeral 24 denotes a condensing lens forcondensing reflected light.

In the conventional moire method using divergent light, an object havinga mirror-reflecting surface such as a glass member or silicon wafercannot be measured because the reflection angle changes in accordancewith the incident angle which changes depending on the position (anobject having a diffused reflecting surface can be measured because theangle seen by an observer becomes a reflection angle). In contrast tothis, according to the parallel light moire method, since an incidentangle and reflection angle remain the same regardless of the position,even a mirror surface object can be measured.

In the phase shift method, assuming that discrete information such ascontour line fringes is a periodic trigonometric function of a lightintensity, the information is handled as continuous information, i.e.,the phase of the trigonometric function, to recognize a surface shapewith a precision higher than the number of contour line fringes. Thephase shift method is disclosed in, for example, Tomizawa and Yoshizawa,“Phase-Shifting Shadow Moire Method”, Proceedings of JSPE Fall Meeting(1991), p. 677.

3. DISCLOSURE OF INVENTION

[Problem to be Solved by the Invention]

As described above, even a mirror surface object can be measured byusing the parallel light moire method. In practice, however, in usingthe parallel light moire method, however, if a measurement target objectis a mirror surface object, and the measurement target object has aninclined surface, a measurement error is caused, and the surface shapecannot be accurately measured.

The reason why such a problem arises will be described below withreference to FIG. 9. As shown in FIG. 9, if the surface of the mirrorsurface object inclines with respect to a horizontal plane (a planeparallel to the grating) by ψ, a normal L1 to the object surfaceinclines by ψ with respect to a normal L0 to the horizontal plane.Therefore, the direction of reflected light at the time of incidence ofparallel light on a horizontal plane (to be referred to as inclinedsurface reflected light hereinafter) shifts by 2ψ with respect toreflected light at the time of incidence of parallel light on ahorizontal plane (to be referred to as a horizontal plane reflectedlight hereinafter).

In this case, a distance a between an incident point on the objectsurface and a point on the grating surface which horizontal planereflected light reaches is obtained bya=H tan θ  (2)where H is the distance between the object surface (incident point) andthe grating. In addition, a distance a′ between an incident point on theobject surface and a point on the grating surface which reflected lightreaches when inclined is obtained bya′=H tan(θ+2ψ)  (3)

According to equations (2) and (3), a difference Δa between thedistances a′ and a is given byΔa=H{tan(θ+2ψ)−tan θ}  (4)

As compared with a case wherein parallel light is incident on ahorizontal plane, when parallel light is incident on an inclined objectsurface, contour lines shift by some pitches. Therefore, a measurementerror δh can be expressed asδh=(Δa/p)Δh  (5)

According to equations (1) and (4), equation (5) can be modified asfollows:δh=H×{tan(θ+2ψ)−tan θ}/(2 tan θ)  (6)

As described above, according to the conventional measurement method, ifa measurement target object is a mirror surface object and has aninclined surface, the measurement error δh is caused. Note that thisproblem is common to the moire method and the oblique incidentinterference method of using interference fringes produced by obliquelyincident light as contour line fringes.

The present invention has been made to solve the above problem, and hasas its object to provide a surface shape measuring system which canaccurately measure the surface shape of a mirror surface object.

[Means of Solution to the Problem]

A surface shape measuring system according to the present inventioncomprises an optical element for formation of contour line fringes whichis disposed to face a measurement target surface of a measurement targetobject, a light source which irradiates the optical element withillumination light, a camera which captures an image of the contour linefringes formed on the optical element by light passing through theoptical element and reflected by the measurement target surface, movingmeans for changing a distance between the optical element and themeasurement target surface, and analyzing means for performing ananalysis process of obtaining 3-D shape information of the measurementtarget surface from an image picked up by the camera in at least twocases in which the distance is set to different values, and obtainingtrue 3-D shape information from which a measurement error caused by aninclination of the measurement target surface is eliminated, on thebasis of the 3-D shape information in each of the cases and thedistance.

In an example of the arrangement of the surface shape measuring systemaccording to the present invention, the optical element is a grating,and the contour line fringes are moire fringes formed by superimposingthe grating and a grating image passing through the grating andreflected by the measurement target surface.

In an arrangement example of the surface shape measuring systemaccording to the present invention, the optical element is a prism, andthe contour line fringes are interference fringes formed bysuperimposing light reflected by a prism surface and light passingthrough the prism and reflected by the measurement target surface.

In an arrangement example of the surface shape measuring systemaccording to the present invention, the analyzing means obtains a linearfunction representing a relationship between the distance and the 3-Dshape information on the basis of the 3-D shape information in at leastthe two cases in which the distance is set to different values, and setsa functional value obtained when the distance is 0 as true 3-D shapeinformation from which the measurement error is eliminated.

4. BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing the arrangement of a surface shapemeasuring system according to an embodiment of the present invention;

FIG. 2 is a flow chart showing the operation of the surface shapemeasuring system in FIG. 1;

FIG. 3 is a graph showing the relationship between the light intensityand the distance from a grating;

FIG. 4 is a graph showing the relationship between the measurement dataand the distance between the grating and the measurement target surface;

FIG. 5 is a graph showing an example of the measurement result obtainedby the surface shape measuring system in FIG. 1;

FIG. 6 is a view for explaining an oblique incident interference method;

FIG. 7 is a view for explaining the reason why a measurement error iscaused in the oblique incident interference method;

FIG. 8 is a view for explaining a conventional parallel light moiremethod; and

FIG. 9 is a view for explaining the reason why a measurement error iscaused on a mirror surface object.

5. BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be described in detail nextwith reference to the accompanying drawings. FIG. 1 is a block diagramshowing the arrangement of a surface shape measuring system according toan embodiment of the present invention. FIG. 2 is a flow chart showingthe operation of the surface shape measuring system in FIG. 1. Thesurface shape measuring system in FIG. 1 includes a light source 1 foroutputting monochromatic point light such as a helium neon laser beam, alens 2 for converting the illumination light emitted from the lightsource 1 into parallel light, a grating 3 which is an optical elementdisposed to be substantially parallel to the measurement target surfaceof a measurement target object 11 placed on a stage 10, a condensinglens 4 for condensing the moire fringe image formed by superimposing thegrating 3 and a grating image passing through the grating 3 andreflected by the measurement target surface of the measurement targetobject 11, a slit 5 for capturing only reflected components from themeasurement target object 11 by eliminating diffracted components fromthe grating 3, a camera 6 for capturing a moire fringe image, an A/Dconverter 7 for converting the image signal output from the camera 6into digital data, an analyzing means 8 for obtaining 3-D shapeinformation of the measurement target surface from the moire fringeimage, and a moving means 9 such as a piezoelectric actuator or steppingmotor which changes the distance between the grating 3 and themeasurement target surface by vertically moving the grating 3 whilekeeping it parallel to the measurement target surface. The grating 3 isformed from a glass plate or the like which has light-shielding portionsarranged at a predetermined pitch p.

The operation of the surface shape measuring system according to thisembodiment will be described below. First of all, the analyzing means 8controls the moving means 9 to move the grating 3 so as to set adistance H between the grating 3 and the measurement target surface ofthe measurement target object 11 to a first predetermined value H1(e.g., 8 mm) (step 101 in FIG. 2).

Subsequently, the analyzing means 8 captures the moire fringe imageformed on the measurement target object 11 (step 102). The illuminationlight emitted from the light source 1 is converted into parallel lightby the lens 2. This parallel light passes through the grating 3 andstrikes the measurement target object 11 placed on the stage 10 to formthe shadow of the grating 3 on the measurement target object 11. As aconsequence, the image of the grating 3 which has passed through thegrating 3, struck on the object 11, and deformed in accordance with thesurface shape of the object 11 is superimposed on the grating 3 to formmoire fringes in the form of contour lines corresponding to the surfaceshape of the object 11.

The moire fringe image is condensed by the condensing lens 4, passesthrough the slit 5, and strikes the camera 6. The camera 6 converts theincident light into an electrical signal. In this manner, the moirefringe image signal is output from the camera 6. This image signal isconverted into digital data by the A/D converter 7. The analyzing means8 captures the image data output from the A/D converter 7 and stores theimage data in an internal memory. With this process, image capturing iscompleted.

After the image capturing, the analyzing means 8 checks whether imagecapturing has been done four times (step 103). In this case, since imagecapturing has not been done four times, the analyzing means 8 controlsthe moving means 9 to move the grating 3 upward by Δh/4 (step 104), andperforms image capturing in step 102 again.

The processing in steps 102 to 104 is repeated until image capturing isdone four times. As a consequence, the image data obtained when thedistance between the grating 3 and the measurement target object 11 isH1, H1+Δh/4, H1+Δh/2, and H1+3Δh/4 are stored in the memory of theanalyzing means 8. Note that an interval Δh between contour line fringescan be calculated by equation (1).

The analyzing means 8 then controls the moving means 9 to move thegrating 3 so as to set the distance between the grating 3 and themeasurement target surface of the measurement target object 11 to asecond predetermined value H2 (e.g., 16 mm) when it is assumed that themeasurement target surface is perfectly flat (step 105). The processingin steps 106 to 108 is the same as that in steps 102 to 104. In additionto the above image data corresponding to four image capturingoperations, the image data obtained when the distance between thegrating 3 and the measurement target object 11 is H2, H2+Δh/4, H2+Δh/2,and H2+3Δh/4 are stored in the memory of the analyzing means 8.

Subsequently, the analyzing means 8 performs the first analysis processof calculating 3-D shape information of the measurement target surfaceof the measurement target object 11 (a relative distance h(X, Y) betweenthe grating 3 and a point with coordinates (X, Y) on the measurementtarget surface) by using the phase shift method for the image datacorresponding to four image capturing operations which are captured inthe processing in steps 101 to 104 (step 109).

A light intensity I(X, Y) of a moire contour line fringe at coordinates(X, Y) on the measurement target surface of the measurement targetobject 11 is a periodic function of the relative distance h(X, Y) withrespect to the grating 3. Assuming that this periodic function is a sinewave, the light intensity is expressed as shown in FIG. 3 and can berepresented asI(X, Y)=a(X, Y)+b(X, Y)×cos(2πh(X, Y)/Δh+φ)  (7)where φ is the phase, a(X, Y) is the light intensity offset, and b(X, Y)is the amplitude of light intensity. These values change depending onlight source intensity irregularity, a flaw on the lens, the patternattached to the measurement target object 11, and the reflectance of themeasurement target object 11.

In order to obtain the relative distance h(X, Y) between the measurementtarget object 11 and the grating 3, the unknowns a(X, Y) and b(X, Y) ofequation (7) must be eliminated. For this purpose, the phase φ ischanged to 0, π/2, π, and 3π/2, and light intensities I0(X, Y), I1(X,Y), I2(X, Y), and I3(X, Y) in the respective cases are obtained. Theselight intensities in these four cases can be expressed as:$\begin{matrix}\begin{matrix}{{{I0}\left( {X,Y} \right)} = {{a\left( {X,Y} \right)} + {{b\left( {X,Y} \right)} \times}}} \\{\cos\left( {2\pi\quad{{h\left( {X,Y} \right)}/\Delta}\quad h} \right)}\end{matrix} & (8) \\\begin{matrix}{{{I1}\left( {X,Y} \right)} = {{a\left( {X,Y} \right)} + {{b\left( {X,Y} \right)} \times}}} \\{\cos\left( {{2\pi\quad{{h\left( {X,Y} \right)}/\Delta}\quad h} + {\pi/2}} \right)} \\{= {{a\left( {X,Y} \right)} - {{b\left( {X,Y} \right)} \times}}} \\{\sin\left( {2\pi\quad{{h\left( {X,Y} \right)}/\Delta}\quad h} \right)}\end{matrix} & (9) \\\begin{matrix}{{{I2}\left( {X,Y} \right)} = {{a\left( {X,Y} \right)} + {{b\left( {X,Y} \right)} \times}}} \\{\cos\left( {{2\pi\quad{{h\left( {X,Y} \right)}/\Delta}\quad h} + \pi} \right)} \\{= {{a\left( {X,Y} \right)} - {{b\left( {X,Y} \right)} \times}}} \\{\cos\left( {2\pi\quad{{h\left( {X,Y} \right)}/\Delta}\quad h} \right)}\end{matrix} & (10) \\\begin{matrix}{{{I3}\left( {X,Y} \right)} = {{a\left( {X,Y} \right)} + {{b\left( {X,Y} \right)} \times}}} \\{\cos\left( {{2\pi\quad{{h\left( {X,Y} \right)}/\Delta}\quad h} + {3{\pi/2}}} \right)} \\{= {{a\left( {X,Y} \right)} + {{b\left( {X,Y} \right)} \times}}} \\{\sin\left( {2\pi\quad{{h\left( {X,Y} \right)}/\Delta}\quad h} \right)}\end{matrix} & (11)\end{matrix}$

The following equation can be obtained by eliminating a(X, Y) bysubtracting equation (10) from equation (8):I 0(X, Y)−I 2(X, y)=2b(X, Y)cos(2πh(X, Y)/Δh)  (12)

The following equation can be obtained by eliminating a(X, Y) bysubtracting equation (9) from equation (11):I 3(X, Y)−I 1(X, Y)=2b(X, Y)sin(2πh(X, Y)/Δh)  (13)

According to equations (12) and (13), the relative distance h(X, Y) canbe obtained byh(X, Y)=(Δh/2π)tan⁻¹{(I 3(X, Y)−I 1(X, Y))/(I 0(X, Y)−I 2(X, Y))}  (14)

According to equation (14), the relative distance h(X, Y) between thegrating 3 and the point with the coordinates (X, Y) on the measurementtarget surface can be obtained without being influenced by differencesin the offset a(X, Y) and amplitude b(X, Y). In order to shift the phaseof the moire fringes π/2 at a time, the distance between the measurementtarget object 11 and the grating 3 is moved Δh/4 at a time, image datacorresponding to four image capturing operations is captured, the lightintensity I(X, Y) of each image data is obtained for each set ofcoordinates X and Y, and the relative distance h(X, Y) is obtained byequation (14).

In this case, the light intensity obtained from the image data obtainedwhen the distance between the grating 3 and the measurement targetobject 11 is H1 is the light intensity I0(X, Y) when the phase φ is 0.The light intensities obtained from the image data corresponding to thedistances H1+Δh/4, H1+Δh/2, and H1+3Δh/4 are the light intensities I1(X,Y), I2(X, Y), and I3(X, Y) when the phase φ is π/2, π, and 3π/2.Therefore, the relative distance h(X, Y) can be obtained from the imagedata which correspond to four image capturing operations and arecaptured in the processing in steps 101 to 104.

The analyzing means 8 then performs the second analysis process ofcalculating the 3-D shape information of the measurement target surfaceof the measurement target object 11 by using the phase shift method forthe image data which correspond to four image capturing operations andare captured in the processing in steps 105 to 108 (step 110). Thesecond analysis process can be done in the same manner as the firstanalysis process.

That is, the light intensities obtained from the image data obtainedwhen the distance between the grating 3 and the measurement targetobject 11 is H2, H2+Δh/4, H2+Δh/2, and H2+3Δh/4 are the lightintensities I0(X, Y), I1(X, Y), I2(X, Y), and I3(X, Y) when the phase φis 0, π/2, π, and 3π/2. The relative distance h(X, Y) can therefore beobtained from the image data which correspond to four image capturingoperations and are captured in the processing in steps 105 to 108 byusing equation (14).

The analyzing means 8 then eliminates a measurement error δh caused bythe inclination of the measurement target surface from the relativedistances h(X, Y) calculated in steps 109 and 110 (step 111). Asindicated by equation (6), the measurement error δh is proportional tothe distance H between the grating 3 and the measurement target object11. Therefore, letting h1(X, Y) be the distance calculated in step 109from the image data corresponding to four image capturing operations insteps 101 to 104, and h2(X, Y) be the distance calculated in step 110from the image data corresponding to four image capturing operations insteps 105 to 108, a true measurement value h0(X, Y) after theelimination of the measurement error δh can be given byh 0(X, Y)=h 1(X, Y)−h 1(X, Y)×(h 1(X, Y)−h 2(X, Y))/(H 1−H 2)  (15)

FIG. 4 shows the relationship represented by equation (15). Themeasurement value h0(X, Y) is the value when the distance H between thegrating 3 and the measurement target object 11 is 0. As a condition inwhich equation (15) holds, it is required that the distances H1 and H2be sufficiently large with respect to the maximum value (e.g., about 10to 100 μm) of the level differences on the measurement target surface ofthe measurement target object 11.

The distances H1 and H2 are measured by a mechanical detector mounted onthe moving means 9, and hence have measurement errors. For this reason,H2 is set to be about two to three times H1, so the difference betweenH1 and H2 becomes sufficiently large with respect to the maximum valueof the level differences. Another condition for the determination of thedistances H1 and H2 is that the contrast of moire fringes is clear. Inconsideration of the above conditions, in this embodiment, H1 is set to8 mm, and H2 is set to 16 mm.

In this manner, the true height h0(X, Y) of the object surface after theelimination of the measurement error δh can be calculated by equation(15).

FIG. 5 shows an example of the measurement result obtained by thesurface shape measuring system according to this embodiment. FIG. 5shows the result obtained by measuring the shape of the concave surfaceof a concave mirror as the measurement target object 11, which has aradius of 30,000 mm. As is obvious, although the measurement dataobtained when the distance H is 8 mm and 16 mm greatly deviate from theideal curve, the data after error correction in step 111 is broughtclose to the ideal curve.

In this embodiment, error correction according to the present inventionis applied to a combination of the parallel light moire method and thephase shift method. However, the present invention may be applied to theoblique incident interference method. As shown in FIG. 6, in the obliqueincident interference method, a prism 12 is disposed as an opticalelement to face the measurement target surface of the measurement targetobject 11. The prism 12 is irradiated with illumination light. The lightreflected by the prism surface is then superimposed on the light whichpasses through the prism 12, is reflected by the measurement targetsurface, and strikes the prism 12 again, thereby forming interferencefringes on a screen 13. These interference fringes are picked up by acamera 14 to obtain 3-D shape information on the basis of theinterference fringe image.

In this oblique incident interference method, the phase differenceobtained when the measurement target surface of the measurement targetobject 11 inclines from a horizontal plane by ψ becomes the measurementerror δh. This measurement error δh is expressed asδh=(2π/λ){(AC−AB)}−nDC)  (16)where λ is the wavelength of incident light, n is the degree, and AC,AB, and DC are the distance between a point A and a point C, thedistance between the point A and a point B, and the distance between apoint D and the point C in FIG. 7. The distances AC, AB, and DC can beobtained as follows:AC={1/cos(θ′+ψ)}H  (17)AB={1/cos θ′)H  (18)DC=BC sin(90−θ)  (19)According to equation (19), the distance BC between the point B and thepoint C is obtained by BC=H{tan(θ+2ψ)−tan θ}  (20)

As described above, since the measurement error δh in the obliqueincident interference method is proportional to the distance H betweenthe prism 12 and the measurement target surface (point A) of themeasurement target object 11, the measurement error can be eliminated byapplying the present invention.

In addition, in this embodiment, the 3-D shape information h1(X, Y) and3-D shape information h2(X, Y) are obtained when the distance betweenthe grating 3 and the measurement target object 11 is H1 and H2, and thetrue 3-D shape information h0(X, Y) is obtained from these pieces ofinformation. However, the distance H may be set to three or more values.In this case, 3-D shape information may be obtained for each distance Hin the same manner as in the first embodiment, a linear function (afunction representing the straight line in FIG. 4) representing therelationship between the distance H and the 3-D shape information may beobtained from these pieces of 3-D shape information by using the leastsquares method, and a functional value when the distance H is 0 may beobtained as the true 3-D shape information h0(X, Y) from this function.

In addition, in this embodiment, the measurement target surface of themeasurement target object 11 is substantially parallel to the grating 3.In practice, however, the grating 3 has a slight inclination withrespect to the measurement target surface (for example, a height ofabout 100 μm with respect to the measurement target object 11 100 mmsquare). This is because it prevents reflected/diffracted light from thegrating 3 from striking the camera 6. Reflected/diffracted light fromthe grating 3 is offset from the optical path of reflected/diffractedlight from the measurement target surface of the measurement targetobject 11 and shielded by the slit 5. This light is not thereforeprojected on the camera 6. Note that since the inclination of thegrating 3 is small, it has no influence on the calculation in step 111.

In this embodiment, a helium neon laser as a monochromatic light sourcehaving a short coherent length is used as the light source 1. However,the present invention is not limited to this. A combination of sodiumlamp or mercury lamp as an incoherent light source and a filter thattransmits only a specific emission spectrum may be used.

As has been described above, according to this embodiment, an analysisprocess of obtaining the 3-D shape information of a measurement targetsurface from the image picked up by the camera 6 is performed in atleast two cases in which the distance between an optical element and themeasurement target surface is set to different values, and a computationis performed on the basis of the 3-D shape information obtained in eachcase and the distance, thereby obtaining true 3-D shape information fromwhich the measurement error caused by the inclination of the measurementtarget surface is eliminated. As a consequence, the surface shape of amirror surface object can be accurately measured.

6. INDUSTRIAL APPLICABILITY

As described above, the present invention is suitable for themeasurement of the surface shape of a mirror surface object.

1. A surface shape measuring system characterized by comprising: anoptical element for formation of contour line fringes which is disposedto face a measurement target surface of a measurement target object; alight source which irradiates said optical element with illuminationlight; a camera which captures an image of the contour line fringesformed on said optical element by light passing through said opticalelement and reflected by the measurement target surface; moving meansfor changing a distance between said optical element and the measurementtarget surface; and analyzing means for performing an analysis processof obtaining 3-D shape information of the measurement target surfacefrom an image picked up by said camera in at least two cases in whichthe distance is set to different values, and obtaining true 3-D shapeinformation from which a measurement error caused by an inclination ofthe measurement target surface is eliminated, on the basis of the 3-Dshape information in each of the cases and the distance.
 2. A surfaceshape measuring system according to claim 1, characterized in that saidoptical element is a grating, and the contour line fringes are moirefringes formed by superimposing said grating and a grating image passingthrough said grating and reflected by the measurement target surface. 3.A surface shape measuring system according to claim 1, characterized inthat said optical element is a prism, and the contour line fringes areinterference fringes formed by superimposing light passing through saidprism and reflected by the measurement target surface and lightreflected by a prism surface.
 4. A surface shape measuring systemaccording to claim 1, characterized in that said analyzing means obtainsa linear function representing a relationship between the distance andthe 3-D shape information on the basis of the 3-D shape information inat least the two cases in which the distance is set to different values,and sets a functional value obtained when the distance is 0 as true 3-Dshape information from which the measurement error is eliminated.